Study of antibacterial activity of Lactococcus lactis sp. against spoilage psychrotrophic bacteria isolated from refrigerated raw milk.

Effect of Lactococcus lactis ssp. lactis inoculated in UHT skim milk on the proteolytic activity of psychrotrophic strains was studied. The titration of a purified non nitrogenous fraction from refrigerated UHT milk samples, previously inoculated by 106 psychrotrophic strains, showed that Pseudomonas fluorescens and P. putida were the most proteolytic species. This result has also been confirmed using polyacrylamide gel electrophoresis of casein fractions extracted from refrigerated and contaminated UHT milk samples. All extracellular proteases of the studied germs revealed a decreasing affinity towards κ, β, and αs1-caseins. This bacterial proteolysis of the UHT milk caseins samples, voluntarily contaminated by each of the studied psychrotrophic strains, was significantly reduced following the addition of 106 CFU/ml Lactococcus, which caused, after 9 days of refrigeration, an appreciable reduction of the number of psychrotrophic germs from a minimum of 100 to 1000 times, depending on the tested strain. Our results suggest that the inhibitory effect of lactic acid bacteria on psychrotrophs is due to acidification or/and bacteriocin like production.